In classical layer 1 transport networks employing time-division multiplexing (TDM) and/or wavelength division multiplexing (WDM), such as the Optical Transport Network (OTN) or the Synchronous Digital Hierarchy (SDH), techniques have been established to support dynamic resource allocation and setup of traffic paths. A transport network, which allows traffic paths to be set up through a switched network automatically is termed Automatically Switched Transport Network (ASTN) or Automatically Switched Optical Network (ASON). The WDM layer is often also called layer 0.
Traditionally, creating traffic paths through a series of Network Elements has involved configuration of individual cross-connections on each network element. ASTN/ASON allows the user to specify the start point, end point, and bandwidth required, and the ASTN/ASON agent on the network elements will allocate the path through the network, provision the traffic path, set up cross-connects, and allocate bandwidth from the paths for the user requested service. The actual path that the traffic will take through the network is not specified by the user.
A protocol suite known as GMPLS (Generalized Multiprotocol Label Switching) has been developed to dynamically provision resources and to provide network survivability using protection and restoration techniques. GMPLS includes three main protocols: a signaling protocol known as the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE), a routing protocol known as the Open Shortest Path First with Traffic Engineering extensions (OSPF-TE), and the Link Management Protocol (LMP).
The underlying architecture of an ASTN/ASON assumes a forwarding (or data) plane and a separated control plane, where each of the network layers can use physically diverse data or forwarding planes. The focus of GMPLS is on the control plane.
Furthermore, the concept of Virtual Private Networks (VPNs) has been extended to layer 1 networks, recently. A Layer 1VPN (L1VPN) is a service offered by a core layer 1 network to provide layer 1 connectivity between two or more customer sites, and where the customer has some control over the establishment and type of the connectivity (see IETF RFC 4847).
GMPLS defines both routing and signaling protocols for the creation of Label Switched Paths (LSPs). In a peer model, edge nodes support both a routing and a signaling protocol. The protocol interactions between an edge node and a core node are the same as between two core nodes. In the overlay model, however, the core nodes act more as a closed system. The edge nodes do not participate in the routing protocol instance that runs among the core nodes; in particular, the edge nodes are unaware of the topology of the core nodes (see IETF RFC 4208).
In the overlay model, customer edge (CE) devices are connected over a User-Network Interface (UNI) to provider edge (PE) devices. PE devices are connected to other provider (P) devices via a Network-Network Interface (NNI).
Current UNIs include features to facilitate requests for end-to-end (i.e. CE to CE) services that include the specification of constraints such as bandwidth requirements, protection needs, and (of course) destinations. Current NNIs on the other hand include features to exchange routing information, as well as to facilitate requests for end-to-end services.
If one CE device is connected to two PE devices, this configuration is referred to as dual-homing. Dual-homing is typically used to avoid a single point of failure (e.g. UNI link or PE) or if two disjoint connections will form a protection group. Route diversity for LSPs from dual-homed CEs is a common requirement in optical transport networks.